This application claims priority to an application entitled xe2x80x9cApparatus and Method for Synchronizing Channels in W-CDMA Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Apr. 29, 1999 and assigned Serial No. 99-15332, and an application filed in the Korean Industrial Property Office on May 25, 1999, and assigned Serial No. 99-18921, the contents of both of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a synchronizing device and method for a CDMA (Code Division Multiple Access) communication system, and in particular, to a device and method for synchronizing channels in a W-CDMA (Wideband-CDMA) communication system.
2. Description of the Related Art
Next generation W-CDMA mobile communication systems assign unique base station codes to each base station to perform asynchronous operation between the base stations. For 512 cells, 512 unique codes are assigned to identify 512 base stations. In such an asynchronous mode base station communication system, a mobile station detects the base station signal being currently received at the highest power, in order to successfully perform a call. However, in the asynchronous base station system, it takes quite a long time to examine the phases of all the possible codes in the cell search, so that it is difficult to apply a general cell search algorithm. Therefore, a multi-step cell search algorithm has been proposed. This method classifies 512 cells into 32 groups and each group includes 16 cells. To employ this method, sync channels are used which include a primary sync channel signal (code) and a secondary sync channel signal (code).
FIG. 1 shows a sync channel structure used for cell search in an asynchronous W-CDMA system. In FIG. 1, reference numeral 1-1 denotes a primary sync channel (PRIMARY SCH) signal, reference numeral 1-3 denotes a secondary sync channel (SECONDARY SCH) signal, and reference numeral 1-5 denotes a common pilot channel signal. One frame has 16 slots. The primary sync channel signal and the secondary sync channel signal are transmitted for a N-chip (256 chip) length at the starting point of every slot. Orthogonality between the two channel signals is maintained so that they can be transmitted at the same time. Further, the common pilot channel uses a unique PN (Pseudo Noise) code (spreading code) for each base station, and the period of the PN code is identical to one-frame length.
The W-CDMA system having the above channel structure uses Gold codes of period 218xe2x88x921 for the unique PN codes, and uses only M (=512) codes out of all possible Gold codes of that length. The common pilot channel signal is not transmitted simultaneously with the primary sync channel signal and the secondary sync channel signal, but only transmitted at other time periods.
The sync channels use sync codes, and the sync codes are generated by performing modulo operation between a Hadamard sequence and a hierarchical sequence. The hierarchical sequence y is generated using a sequence x1 of length n1 and a sequence x2 of length n2, as follows:
xe2x80x83y(i)=x2(imodn2)+x1(i÷n1) for i=0, . . . , (n1*n2)xe2x88x921
Further, the sequences xi and x2 are select sequences of length 16 as follows.
X1= less than 0,0,1,1,0,1,0,1,1,1,1,1,0,0,0,1 greater than 
X2= less than 0,0,1,1,1,1,0,1,0,0,1,0,0,0,1,0 greater than 
The Hadamard sequences are obtained as the rows in a matrix H8 constructed recursively by:             H      k        =          (                                                  H                              k                -                1                                                                        H                              k                -                1                                                                                        H                              k                -                2                                                                        H                              k                -                1                                                        )        ,      xe2x80x83    ⁢      k    ≥    1  
The rows are numbered from top starting with row 0 (the all ones sequence). The nth Hadamard sequence is denoted as the nth row of H8 numbered from the top, n=0,1,2, . . . ,255, in the sequel.
Therefore, let hm(i) and y(i) denote the ith symbol of the sequence hn and y, respectively where i=0, 1, 2 . . . , 255 and i=0 corresponds to the leftmost symbol.
By XOR-gating a 256-chip Hadamard sequence hm(i) and the hierarchical sequence y(i), a kth sync code is then defined as
Csc, k={hm(0)+y(0), hm(1)+y(1), hm(2)+y(2), . . . , hm(255)+y(255)},
Where m=8xc3x97k, k=0,1,2, . . . , 17, and the leftmost chip in the sequence corresponds to the chip transmitted first in time.
Then, synchronization code #0 generated in the above manner is assigned to the P-SCH signal, where
Cp=Csc, 0
The other synchronization codes, Csc, 1to Csc, 17 are assigned in the respective slots of a secondary sync (S-SCH) signal.
The primary sync code cp is repeatedly transmitted only for 256 chips every slot, which is {fraction (1/10)} of one slot. The sync code used for the primary sync channel signal is the same for every cell. The primary sync channel signal is used for detecting the slot timing of the received signal by the mobile station. The base station transmitter introduces a comma-free code when transmitting the secondary sync channel. The comma-free code is comprised of 32 code words, and each code word is comprised of 16 symbols and transmitted repeatedly in every frame. However, the 16 symbol values are not transmitted as they are, but each symbol value is mapped into a secondary sync code and is transmitted for frame synchronization and base station group information. The mobile stations have the comma free code table and know the mapping relation of the symbols and secondary sync codes. As shown in FIG. 1, an ith sync code, corresponding to a symbol value xe2x80x98ixe2x80x99, is transmitted every slot. Csi,k indicates the ith secondary synchronization code inserted in the kth slot. The 32 code words of the comma-free code identify 32 groups, and the comma-free code has a unique cyclic shift feature for each code word. Therefore, it is possible to obtain information about the code groups and frame synchronization using the secondary sync channel signal (code). Here, xe2x80x9cframe synchronizationxe2x80x9d refers to synchronization of timing or phase within one period of a PN spreading code in a spreading spectrum system. However, in the existing W-CDMA system, since both one period of the spreading code and the frame length are equal to 10 ms, this PN code synchronization will be referred to as frame synchronization.
In the mobile station, a correlation value is calculated for a spreading code of a base station in order to distinguish different base station codes used by different base stations. Forward common channels, such as a pilot channel and a broadcasting channel (BCH), can be used when calculating the correlation value for the spreading code of the base station. In the conventional W-CDMA system, the pilot symbol is transmitted on the broadcasting channel using Time Division Multiplexing (TDM). However, the recent harmonization group OHG (Organized Harmonization Group) recommends transmitting the forward common pilot. FIG. 1 shows an example where the forward common pilot channel is transmitted by CDM (Code Division Multiplexing) and transmission of the pilot channel is discontinued when the sync code is transmitted.
FIG. 2 shows an example where the forward common pilot channel signal is transmitted by CDM and the pilot channel signal is continuously transmitted without discontinuation even when the sync channel signals are transmitted.
The common pilot channel signal can transmit the pilot symbol and data using time division multiplexing in every slot (the existing W-CDMA structure). Otherwise, there may be provided separate channels for transmitting the data. In this case, the channel frame for transmitting data should have the same boundary as the common pilot channel frame. Generally, the common pilot channel does not transmit data, but only transmits the pilot symbol, all +1 or xe2x88x921.
In the synchronization process of the conventional W-CDMA system, the synchronization is acquired through three search steps. In the first step, synchronization of the 0.625 ms slot is acquired. In the second step, frame synchronization is acquired and group identification is performed. In the third step, the spreading code (the specific base station code) used in the group is decided.
However, in the conventional synchronization process, when performing the frame synchronization and group identification of the second step, the secondary sync channel is undesirably monitored for a period of 10 ms. That is, in a conventional CDMA communication system, it is not possible to acquire frame synchronization within one period of the spreading code. Further, in the W-CDMA communication system, it is not possible to perform synchronous communication using only one sync channel. Therefore, in a conventional CDMA communication system, the frequent repetitions of transmitting the sync code makes it impossible to minimize interference on the forward link. Accordingly, it is not possible to increase the system capacity.
In addition, in the conventional system, in order to synchronize information about a code group with a frame, the secondary sync channel must be received continuously during one frame. The present invention aims at reducing the overall synchronization time by minimizing the time required for receiving the secondary sync channel.
It is, therefore, an object of the present invention to provide a device and method for minimizing communication of the sync channel signal while acquiring synchronization in a W-CDMA communication system.
It is another object of the present invention to provide a synchronization device and method for a base station in a W-CDMA communication system, wherein a primary sync code for frame synchronization is transmitted at a predetermined location within a one frame period, and a secondary sync code corresponding to the code group to which the base station belongs is transmitted at a location predetermined chip size distance from the primary sync code.
It is further another object of the present invention to provide a synchronization device and method for a base station in a W-CDMA communication system, wherein a primary sync code for frame synchronization is transmitted at a predetermined location within a one frame period, and a secondary sync code corresponding to the code group to which the base station belongs, is transmitted at a previously set location after the primary sync code.
It is yet another object of the present invention to provide a synchronization device and method for a base station in a W-CDMA communication system having a plurality of antennas which support a transmit diversity function, wherein a primary sync code for frame synchronization is transmitted through the antennas at a predetermined location within a one frame period, and a secondary sync code corresponding to the code group, to which the base station belongs, is transmitted at a previously set location after the primary sync code.
It is still another object of the present invention to provide a synchronization, device and method for a base station in a W-CDMA communication system, wherein every base station uses the same spreading code instead of a secondary sync channel code, and each base station transmits a primary sync channel code at a predetermined offset location within a frame length.
It is yet still another object of the present invention to provide a synchronization device and method for a mobile station in a W-CDMA communication system having a base station which transmits a primary sync channel code for frame synchronization at a predetermined location, which is known at the mobile station by a predetermined mobile communication standard, within a one frame period and transmits a secondary sync channel code corresponding to the code group, to which the base station belongs, at a location a predetermined chip size distance from the primary sync channel code, wherein the synchronization device and method determines whether frame synchronization is acquired or not by acquiring the received primary sync channel code and then determines the secondary sync channel code, after acquisition of the primary sync channel, to determine a code group.
In accordance with one aspect of the present invention, a sync channel transmission device for a base station in an asynchronous CDMA communication system comprises a primary sync channel transmitter for generating a primary sync code to indicate the starting point of one frame, said frame equaling one period of a spreading code of common pilot channel, and transmitting the primary sync code at a first location in the frame; and a secondary sync channel transmitter for generating a secondary sync code assigned to the group of base stations to which the base station belongs, and transmitting the secondary sync code at a second location in the frame.
In accordance with another aspect of the present invention, a sync channel receiving device for a mobile station in an asynchronous W-CDMA system comprises a primary sync channel acquisition decider for acquiring a primary sync channel code received at a first location in a frame, and acquiring synchronization for the starting point of a frame, said frame equaling one period of a spreading code of common pilot channel; and a code group decider receives a secondary sync channel code transmitted at a second location in the frame, and determining the base station group to which the corresponding base station belongs by the distance of the primary synchronization code and secondary synchronization.